Frequency conversion system for an electronic musical instrument

ABSTRACT

A frequency conversion system for an electronic musical instrument. The system has a signal source, at least one frequency converter and an electro-acoustic transducer, and converts the fundamental frequency and overtones of each signal generated by said signal source into other frequencies so as to change said signal with said fundamental frequency and overtones into another signal having an inharmonicity function other than and having a natural tone quality, clearness and strength and also to change the pitch of said signal into another pitch which matches the natural pitch sensation recognized by human ears.

O Umted States Patent 1191 1111 3,916,752 Kawamoto et al. Nov. 4, 1975FREQUENCY CONVERSION SYSTEM FOR 3,004,460 10/1961 Wayne 84/1.0l ANELECTRONIC MUSICAL INSTRUMENT 3,007,361 11/1961 Wayne 84/124 X 3,086,1224/1963 Jones 34/DIG. 4 Inventorsr Kinji Kflwamfitoa y g 3,215,76711/1965 Martin 84/1.24 Masahiko Tsunoo, Osaka;.Masuo 3,267,199 8/1966Hurvitz 84/125 Omura, Hirakata, all of Japan 3,418,418 12/1968 Wilder84/125 4 2 1 Assignw Matsushita Electric Industrial 33252:??? 31338$323131: jijjiiiji....2;f. Osaka, Japan 3,598,893 8/1971 Uchiyama 84/124x [22] Filed: Aug. 27, 1971 Primary Examiner.loseph W. Hartary [21] PP-Z 175,714 Assistant Examiner-U. Weldon Related s Application DataAttorney, Agent, or Firm-Wenderoth, Lind & Ponack 63 Continuation ofSer. No. 865 abandoned. Oct 1969 [57] ABSTRACT A frequency conversionsystem for an electronic musi [30] Foreign Application Priority Data calinstrument. The system has a signal source, at least Oct. 21, 1968 Japan43-77565 one frequency Convener and an eleetre-aeeuetie transducer, andconverts the fundamental frequency 52 us. c1. 84/1.19; 84/101; 84/125and overtones of each Signal generated by Said Signal [51] Int. Cl. GIOH3/00 Source into other frequencies 50 as to Change Said [58] Field6rsea1-ch..... 8 1/101, 1.24, 1.25, DIG. 4; ml with Said fundamentalfrequency and Oveflones 331/45; 332/17 23 into another signal having aninharrnonicity function other than and having a natural tone quality,cleamess [56] References Cited and strength and also to change the pitchof said signal UNITED STATES PATENTS into another pitch which matchesthe natural pitch sensation recognized by human ears. 1,788,362 l/1931Young 331/45 X 2,916,706 12/1959 Timperman 84/124 x 7 Claims, 18 DrawingFigures 1 98 r; zoo 3 TQNE FREQ. 7

GEN. cowv.

Hana 1' j U.S. Patent Nov. 4, 1975 Sheet 2 of3 3,916,752

2 3 4 5 FREQUENCY (BEFORE CONVERSION) FIG.|I

I INVENTORS KINJI KAWAMOTO 2 1 MASAHIKO TSUNOO MASUO OMURA ma ATTORNEYSFREQ- CONV.

259m w zutzmfix :11: w w 23 wzok rw l HHH PHIL FREQUENCY CONVERSIONSYSTEM FOR AN ELECTRONIC MUSICAL INSTRUMENT This application is acontinuation of application Ser. No. 865,030, filed Oct. 9, 1969, nowabandoned.

FIELD OF THE INVENTION The present invention relates generally to thetone quality of an electronic musical instrument and more particularlyrelates to a frequency conversion system for getting a new signal thefundamental frequency and overtone frequencies of which are different inharmonicity and pitch from those of the original tone.

DESCRIPTION OF THE PRIOR ART A conventional electronic musicalinstrument has been said to be inferior in tone quality to aconventional musical instrument which utilizes mechanical vibration oracoustical vibration. The difference in tone character between anelectronic musical instrument and a conventional musical instrumentexists for the following reasons. The mechanical vibrations and theacoustical vibrations in a conventional musical instrument generally donot have exact harmonics; that is, the frequencies of the overtones ofthe vibration are not exactly positive integer multiples of thefundamental frequency of the vibration.

For example, the fundamental frequency f and the frequencies of theharmonics f,, due to the vibration of a piano string can be representedas follows,

C /P where l is the free length of the string T is the tension thereof pis the density thereof S is the cross section thereof Q is Youngsmodulus k is the radius of gyration about the neutral axis of the crosssection n is the order of the harmonics (n=1, 2, 3,

We can define an inharmonicity function Y(n) as follows,

where Y(n) is a measure of inharmonicity and a ratio of n-th overtonefrequency f, to the multiple n of the fundamental frequency f,. As brepresented by Equation (5) is generally l0 -l0 so Y(n) becomes no lessthan 4 percent for a large n.

Equation (4) shows that the greater the order of harmonics, the greaterthe inharmonicity.

On the other hand, a sound or a signal of a conventional electronicmusical instrument generated by an electronic oscillator circuit or afrequency divider circuit has exact harmonics, namely, frequencies ofthe overtones f are exactly positive integer multiples of thefundamental frequency f,. The inharmonicity function Y(n) for such asituation is as follows,

The conventional electronic musicalinstrument has another defect in thatthe pitch of the signal does not agree with the natural musical scale.Signals with an exact frequency ratio 2:1 do not sound as though theyare at a one octave musical interval to the human ear, especially in alow or a high frequency range but there must be a frequency ratiogreater than 2:1 for obtaining a l octave musical interval. In aconventional electronic musical instrument, the frequency ratio of twosignals which are at a l octave musical interval is exactly 2:1 from thelow frequency range to high frequency range.

As a result, a conventional electronic musical instrument has the defectthat the pitch of the signal is not sufficiently high in the highfrequency range and is not sufficiently low in the low frequency region.

When tone generators are used for every tone signal corresponding toevery pitch of the natural musical scale, the defect mentioned-above isalmost eliminated, although such a method is unnatural because the exactharmonics of each signal are not actually changed into the inexactharmonics of a conventional instrument. Moreover such construction isexpensive, bulky and hard to maintain.

SUMMARY OF THE INVENTION It is a primary object of the present inventionto pro vide a frequency conversion system for an electronic musicalinstrument, said frequency conversion system acting to improve the tonequality of a conventional electronic musical instrument with respect tonaturalness, clearness and strength by converting an original signalinto a new signal in which the inharmonicity function is other than 1.

It is another object of the invention to provide a frequency conversionsystem which converts an original signal of an electronic musicalinstrument into a new signal in such a way that the ratios of thefrequencies of overtones to the fundamental frequency of the new signalare increased or decreased.

A further object of the present invention is to provide a frequencyconversion system which improves the pitch of a conventional electronicmusical instrument in the low and high pitch regions of the musicalscale to reduce the pitch deviation from the natural pitch.

These objectives are achieved by employing a frequency conversion systemfor electronic musical instrument according to the present invention,which comprises a signal source, at least one frequency converter and anelectro-acoustic transducer which are connected in series to each otherin the described order. A signal from said signal source has thefrequency and the pitch converted by said frequency converter and istransduced into a sound by said electro-acoustic transducer.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and particulars of thepresent invention will be made clear from the following detaileddescription of the invention considered together with the accompanyingdrawings, wherein: I

FIG. 1 is a schematic block diagram of an embodiment of a frequencyconversion system for an electronic musical instrument according to thepresent invention;

FIG. 2 is a diagram which depicts an example of the spectrum, i.e. thefundamental frequency and overtone frequencies, of an original signaland a signal which has had'the frequency converted;

FIG. 3 is a diagram of the inharmonicity function Y(n) vs. the order ofharmonics;

FIG. 4 is a schematic block diagram of a second embodiment ofa frequencyconversion system for an electronic musical instrument according to thepresent invention; I

FIG. Sis a schematic block diagram of a third embodiment ofa frequencyconversion system for an electronic musical instrument according to thepresent invention;

FIG. 6 is a schematic block diagram of a fourth embodiment of a'frequency conversion system for an electronic'musical instrumentaccording to the present invention; I I

FIG. 7 is a schematic block diagram of a fifth embodiment of a frequencyconversion system for an electronic'musical instrument according to thepresent invention;

FIG. 8 is a diagram of the characteristic of frequency vs. pitch;

FIG. 9 is a schematic block diagram of a sixth embodiment of a frequencyconversion system for an electronic musical instrument according to thepresent invention;

FIG. 10 is a schematic block diagram of a seventh embodiment of afrequency conversion system for an electronic musical instrumentaccording to the present invention; I I

FIG. 11 is a diagram of the frequency conversion characteristic of theembodiment shown in FIG. 10;

FIG. 12 is a schematic block diagram of an example of a frequencyconverter which is used in a frequency conversion system for anelectronic musical instrument according to the present invention;

FIG. 13 is a schematic block diagram of a multiplier which is used inthe frequency converter shown in FIG. 12;

FIG. 14 is a schematic block diagram of another example of a frequencyconverter which is used in a frequency conversion system for anelectronic musical instrument according to the present invention; and

FIGS. ISA-D is a diagram of a frequency spectrum for explaining theoperation of the frequency converter shown in FIG. 14.

DETAILED DESCRIPTION FIG. 1 is a diagram ofa monophonic electronicmusical instrument which can produce a new tone.

Referring to FIGS. 1, '2, and 3, in FIG. 1, a signal source 100comprises a tone generator 98 and a pitch selector 1. The tone generator98 produces a signal 6 which has fundamental and exact harmonics, i.e.,a frequency spectrum f,,f ,f ,f,,, where the fundamental and harmonicfrequencies are f Hz, f Hz, f Hz, ...,f,.Hz,...and

as shown by solid lines in FIG. 2. The fundamental frequency f iscontrolled by a pitch selector 1. A frequency converter 200 subtractsAfl-Iz from the signal 6 and produces a signal 7 having frequencycomponents (fr-AflHz, (Zfl-AflHz, (3f,Af)Hz, (nf,Af)Hz,

. as shown by dotted lines in FIG. 2. The signal 7 is transduced into asound by an electro-acoustic transducer 3.

The sound has such new tone qualities as naturalness, clearness andstrength, and is a new tone which can not be obtained by a conventionalelectronic musical instrument. I

As for the signal 6, the inharmonicityfunction Y(n) Therefore the signal6 does not have a naturalness of a tone produced by a conventionalmusical instrument. This relationship is shown by a solid line 10 inFIG. 3.

As for the signal 7, the inharmonicity function Y(n) can be representedas follows,

where (f Af) is a fundamental frequency and (nf -Af) is the frequency ofthe n-th harmonic. Equation (8) indicates that the ratios of thefrequencies of overtones to the fundamental frequency increase with anincrease in the frequency. Equation (8) is shown by a dotted line 11 inFIG. 3. The dotted line 11 shows that a signal 7 has an inharmonicityfunction somewhat similar to a piano tone, the inharmonicity functionY(n) of which is represented by Equation (4) and is shown by the curvedline 12 in FIG. 3. The frequency Afis preferably a sub-audio frequency,for example, several Hz, for simulation of a piano tone.

When the frequency Af is less than 2 percent of the fundamentalfrequencyf, the converted signal 7 has an improved tone quality such asimproved naturalness, clearness and strength. When Afis more than 2percent and less than 4 percent of the fundamental frequency f, the tonequality of the converted signal 7 is further improved with respect tonaturalness, clearness and I strength. When Afis more than 4 percent ofthe fundamental frequency f, the signal 7 has a tone quality that cannotbe obtained either by a conventional electronic musical instrument or bythe conventional musical instrument. I

As mentioned-above, a frequency conversion system according to thepresent invention can change a signal with exact harmonics into anothersignal with inexact harmonics such as those of the natural sound of aconventional musical instrument.

When the frequency converter 200 adds Af'I-Iz to the signal 6 in FIG. 1,the converted signal 7 becomes a signal with inexact harmonics and aninharmonicity function Y(n) represented by Equation (9).

Equation (9) shows that the ratios of the frequencies of overtones tothe fundamental frequency decrease. Equation (9) is shown by curved line13 in FIG. 3. This signal is a new one which a conventional electronicmusical instrument cannot produce.

FIG. 4 shows a second embodiment of a frequency conversion system for anelectronic musical instrument according to the present invention. Asignal source 100 is composed of a tone generator 98 and a pitchselector 1. The tone generator 98 the frequency of which is controlledby the pitch selector 1 produces a signal 6 which has fundamental andexact harmonics. A frequency converter 200 increases or decreases thefrequencies of the signal 6 by Afl-Iz and puts out a signal 7. Thesignal 7 is transduced into a sound by an electro-acoustic transducer 3.Frequency Afl-Iz is also controlled by the pitch selector 1. Then,inharmonicity characteristic of the signal 7 is dependent on the pitchof the signal 7 and an optimum inharmonicity function for the signal 7can be chosen as the pitch of the signal 7 ascends or descends.

FIG. shows the third embodiment of a frequency conversion system for anelectronic musical instrument according to the present invention.Referring to FIG. 5, a signal source 100 comprises tone generators 101,102, 103, 104,. corresponding to the tones ofa musical scale and havefundamental frequencies f flog, f and their exact harmonics,respectively. The tone generatores 101, 102, 103, 104, are connected tofrequency converters 201, 202, 203, 204, respectively. The convertingfrequencies of the frequency converters 201, 202, 203, 204, are Af Af AfAf respectively.

The output signals of the frequency converters 201, 202, 203, 204, areconnected to a keyswitch system 4 of a keyboard. Output signals selectedby the keyswitch system 4 are filtered by a tone filter Sand transducedinto sounds by an electro-acoustic transducer 3.

According to the embodiment shown in FIG. 5, any inharmonicitytransition is obtainable along the musical scale by arranging theconverting frequencies Af Af Af Af for desired inharmonicity transition.For example, when the converting frequencies Af Af Af Af are the samepercentage of the fundamental frequencies of the corresponding tonegenerators, Equation (8) is the same for every tone generator andfrequency converter; in other words, for every tone corresponding to akey of the keyswitch system 4. Therefore, a uniform inharmonicitycharacteristic is obtained over the whole musical scale.

FIG. 6 shows the fourth embodiment of a frequency conversion system foran electronic musical instrument according to the present invention.Referring to FIG. 6, a signal source 100 comprises tone generators 101,I02, I03, 104, corresponding to the musical scale and have fundamentalfrequencies f ,j" ,f ,f and their exact harmonics, respectively. Signalsfrom the tone generator 101, 102, 103, 104, are selected by a keyswitchsystem 4. Output signals from the keyswitch system 4 are gathered in oneor more groups, for example, in three groups, and further fed tofrequency converters 201, 202, and 203 corresponding to the threegroups. Output signals from the frequency converter 201, 202 and 203 arefed to a tone filter 5 and transduced into sounds by electro-acoustictransducer 3. This embodiment requires fewer frequency converters thanthe embodiment shown in FIG. 5 and is more practical.

FIG. 7 shows a fifth embodiment of a frequency conversion system for anelectronic musical instrument according to the present invention toobtain an inharmonicity characteristic in tones. Referring to FIG. 7, asignal source comprises tone generators 101, 102, 103, 104,corresponding to the musical scale and having fundamental frequencies f,f, ,f ,f and their exact harmonics, respectively. Signals from the tonegenerators 101, 102, 103, 104, are selected by a keyswitch system 4 andfed through resistors 301, 302, 303, 304, to a frequency range separator400 comprising filters and separated into a plurality of groups, forexample, into low frequency range signals, medium frequency rangesignals and high frequency range signals. Separated signals have thefrequency converted by frequency converters 201, 202, and 203respectively which are then gathered by gathering means, i.e. resistors501, 502 and 503, and further transduced into sounds by anelectroacoustic transducer 3.

In the embodiments shown in FIG. 5, FIG. 6 and FIG. 7, pitches orfundamental frequencies of the tone generators 101, 102, 103, 104, arepreferably biased for compensation of any unwanted pitch deviatio causedby the frequency conversion.

A frequency conversion system for an electronic musical instrumentaccording to the present invention has another function and effect whichovercomes a defect of the conventional electronic musical instrument,i.e. a discrepancy between the musical scale produced by theconventional electronic musical instrument and a natural pitchsensation.-

FIG. 8 shows an example of the relation between the signal frequency andthe pitch recognized by human ears. As shown in FIG. 8, the sensation ofpitch change saturates in the low and high frequency regions and agreater frequency change is required for human ears to distinguish anequal pitch change in the low and high frequency ranges than is requiredto distinguish a corresponding change in the middle frequency region.

FIG. 9 shows a sixth embodiment of a frequency conversion system for anelectronic musical instrument according to the present invention forcorrecting the pitch imperfection of a conventional electronic musicalinstrument. Referring to FIG. 9, a tone generator 99 of a conventionalelectronic musical instrument generates tone signals which correspond tothe musical scale. Generally, the tone generator 99 is composed oftwelve oscillators corresponding to the respective tones of thechromatic musical scale and twelve chains of frequency dividerscorresponding to said twelve oscillators. For example, when the tonegenerator 99 comprises oscillators and frequency dividers each of whichdivides the frequency of an input signal by a factor of two, the oneoctave musical intervals of the output signals are in the exactfrequency ratio of 2:1.

The signals from the tone generator 99 are fed to a frequency converter200 through a keyswitch 4 and a tone filter 5 and are transduced intosounds by an electro-acoustic transducer 3. The tone generator 99, thekeyswitch system 4 and the tone filter 5 compose a signal source 100.The frequency converter 200 decreases the frequency fHz of the inputsignal by Afl-Iz and produces a signal with a frequency (f-AflHz. Thepercent-' age of frequency change 1 from the original pitchcorresponding to the fundamental frequency fHz of said tone generator 99is represented as follows:

1,: Af/fx 100 1 becomes great as the frequency fHz decreases, i.e., thepitch reduction becomes great as the frequency fHz decreases. Therefore,a tone in the low frequency range has a sufficiently low pitch and thepitch imperfection is improved in the low frequency range. Because theinharmonicity characteristic is also improved, the tone quality isimproved.

FIG 10.'shows a seventh embodiment of a frequency conversion system foran electronic musical instrument according to the present invention forcorrecting the pitch imperfection ofa conventional electronic musicalinstrument. Referring to FIG. 10, a signal source 100 comprises a tonegenerator 99, a keyswitch system 4 and a tone filter 5. The tonegenerator 99 of a conventional electronic musical instrument generatestone signals having frequencies which correspond to the musical scale.The signal from the tone generator 99 is fed to a frequency rangeseparator 40 through the keyswitch system 4 and the tone filter 5, so asto be separated into a plurality of frequency-band-limited signals, forexample, into four signals the range of frequencies of which are (F,-F(F -F (F -F and (F.,-F respectively, where F, F F F F as shown in FIG.10. At least one signal from the frequency range separator 400, forexample, three signals, are fed to sub-frequency converters, forexample, 205, 206 and 207, respectively.

Signals from the sub-frequency converters 205, 206 and 207 and theremainder of the frequency-bandlimited signal which is not fed to anysub-frequency converter are gathered together through resistors 501,502, 503 and 504 and further transduced into sounds by anelectro-acoustic transducer 3.

The frequency range separator 400, the subfrequency converters 205, 206and 207 and resistors 501, 502, 503 and 504 compose a frequencyconverter 210.

When the converting frequencies of the subfrequency converters 205, 206and 207 are Afl-Iz, Af'I-Iz and AfHz, respectively, and are in therelation,

the pitch defect is improved not only in the low frequency range (F,-Fbut also in the high frequency range (F -F.,) and F,-F A negativeconverting frequency corresponds to a frequency decrement and a positiveconverting frequency corresponds to a frequency increment.

FIg. 11 shows an example of the frequency conversion characteristicachieved by the embodiment shown in FIG. 10.

Tone quality is also improved by the inharmonicity characteristic whichis obtained. In the embodiments shown in FIG. 9 and FIG. 10, it is bestif the tone generator 99 is biased to change the frequency forcompensation of unwanted pitch deviation caused by frequency conversion.

When a magnetic tape recorder or a record player for playing recorddiscs is used as a signal source in FIG. 1, FIG. 4, FIG. 9 and FIG. 10,the harmonicity characteristic or the pitch of the signals recorded onthe tape or the disc is changed and the tone quality and the pitchimperfection are improved.

When a conventional musical instrument such as a violin, a clarinet or atrumpet and an acoustic-electric transducer such as a microphone areused as a signal source 100 in FIG. 1, FIG. 4, FIG. 5, FIG. 9 and FIG.10, the inharmonicity characteristic of the musical instrument can bechanged and a new tone having better tone quality than the original tonequality can be obtained. Some conventional musical instruments havingpitch imperfection can be improved. It is also possible to obtain anentirely new tone quality by converting frequencies of signals ofconventional musical instruments.

As mentioned-above, the scope of the present invention is not limited toa particular kind of signal source 100. It is only necessary that thesignal source 100 generate an audio frequency signal.

The frequency converters 200, 201, 202, 203, 204,

.. and sub-frequency converter 205, 206, 207 can be realized as follows.

FIG. 12 shows an example of a frequency converter. Referring to FIG. 12,an audio frequency signal A sin 21rft, such as the output signal ofanelectronic musical instrument, is applied to an input terminal 601 andis split by a constant phase splitter 603 into two signals X and Xhaving a phase difference from each other of -n'/2 radians andrespectively supplied to leads 605 and 606. Signals X and X, from theleads 605 and 606 are fed to multipliers 609 and 610 respectively andare gathered by gathering means, i.e. resistors 611 and 612, andsupplied to an output terminal 613. An oscillator 602 generates a pairof frequency converting signals Y and Y having a phase difference of11/2 radians from each other and supplies them to leads 607 and 608.Signals Y and Y from the leads 607and 608 are fed to the multipliers 609and 610 respectively. The signals X X Y and Y, can be represented asfollows.

X A sin (Z'rrft 4-4)) Y B sin(21rAft +d1) I, B sin(21rAft 1r/2 41)Output signals Z and Z from the multipliers 609 and 610 can berepresented as follows.

2 x AB sin(21rft i 17/2 an sin(211-Aft i 1r/2 l The signals Z and Z aregathered together and a signal /(Z,+Z as represented by Equation (18)appears at the output terminal 613.

The signal V2(Z1+Z2) at the output terminal 613 is a signal thefrequency of which is the sum of or the difference between the twofrequencies from the input terminals 601 and the oscillator 602. Whetherthe frequency of the signal %(Z +Z becomes the sum or the difference ofthe two frequencies depends on the phase dif ference between the twooutput signals from the phase splitter 603 and the phase differencebetween the two output signals from the oscillator 602.

FIG. 13 shows an embodiment of the multipliers 609 and 610. A signal Xapplied to a terminal 614 is modulated in an amplitude modulator 616 bya modulating signal Y applied to a terminal 617. The signal X is alsofed to a phase inverter 615. Output signals from the amplitude modulator616 and the phase inverter 615 are gathered together by gathering means,i.e. resistors 618 and 619, and appear at an output terminal 620. Theoutput signal from the amplitude modulator 616 has a carrier signal Xand its two side bands. When the carrier signal X is cancelled by aphase inverted signal from the phase inverter 615 at the output terminal620, only two side-bands appear at the terminal 620. The two side-bandscorrespond to a signal XY, namely, a product of the signal X and thesignal Y. The multipliers 609 and 610 and the amplitude modulator 616should be linear.

An embodiment of a frequency converter which can be used as thefrequency converter 200, 201, 202, 203, 204, is shown in FIG. 14. Amodulator 623 modulates an audio frequency signal{f which is applied toa terminal 621, by a modulating signal f which has a frequency higherthan {f,,} and which is supplied to another terminal 622. The modulator623 produces at least two side bands fl{j",,} and f +{fl,} as shown inFIG. 15(a) and (b). A band pass filter 624 passes only the lower sideband f {f,,} as shown in FIG. 15(c). Another modulator 626 modulates thefiltered lower side band f {f by another modulating signal )1. (=fg-Aj)and produces at least two side bands f f +{f and f +f {fi,} as shown inFIG. 15(d). The lower side band f '-f +{f is passed by another band passfilter 627 and appears at an output terminal 628. The output signalf f+{f,,} is {fi Afthe frequency of which is lower than that of the signal2} by a small frequency Af. The balanced modulators 623 and 627 can belinear modulators such as multipliers using a Hall effect or nonlinearmodulators such as ring modulators.

While particular embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that numerousmodifications and variations can be made in the form and constructionthereof without departing from the fundamental principles of theinvention.

We claim:

l. A frequency conversion system for an electronic musical instrumentcomprising;

a signal source composed of at least one biased tone generator which isdetuned from a normal musical scale by a predetermined frequencydifference AfHz in a predetermined direction for generating a first tonesignal having a first fundamental frequency component which is biasedfrom said normal musical scale by said frequency difference AfHz in saiddirection and which also has exact harmonic components the frequenciesof which are a positive integer multiple of the frequency of said firstfundamental frequency;

at least one frequency converter connected to said signal source forchanging all the frequency components of said first tone signal in adirection the opposite of said predetermined direction by a convertingfrequency equal to said predetermined frequency difference AjHz in orderto produce a converted tone signal having frequency componentsconsisting of a second fundamental frequency component and inexactharmonics thereof, each of said second fundamental frequency componentand said inexact harmonics having a changed frequency which differs fromsaid first fundamental frequency component and said exact harmonics ofsaid first tone signal by said converting frequency AjHz of saidfrequency converter; and

an electro-acoustic transducer connected to said at least one frequencyconverter.

2. A frequency conversion system for an electronic musical instrument asclaimed in claim 1, wherein:

said signal source consists essentially of twelve master oscillators andtwelve chains of frequency dividers corresponding to said twelve masteroscillators and being coupled thereto, said twelve master oscillatorsbeing biased to bias the frequency from the normal chromatic musicalscale in an upward direction; and said frequency converter connected .tosaid signal source changes the frequency of an output signal from saidsignal source in a downward direction by said converting frequency forcompensating a pitch discrepancy from a natural pitch sensation of thehuman ear.

3. A frequency conversion system for an electronic musical instrument asclaimed in claim 1, wherein said tone generator has a pitch selectorwhich controls both the frequency of said tone generator and saidconverting frequency.

4. A frequency conversion system for an electronic musical instrument asclaimed in claim 1, wherein there is-a plurality of tone generatorscorresponding in number to the respective tones of the musical scale,and there is a plurality of frequency converters corresponding in numberto the number of said tone generators, respectively.

5. A frequency conversion system for an electronic musical instrument asclaimed in claim 1 further comprising a key switch system, and in whichthere is a plurality of tone generators corresponding in number to therespective tones of the musical scale, said key switch system beingconnected to said tone generator for selecting tone signals from amongsaid plurality of tone generators and gathering said selected tonesignals into a plurality of groups each of which has successive tonesignals of the musical scale, said frequency conversion system having aplurality of frequency converters corresponding in number to saidplurality of groups of tone signals, said key switch system beingconnected to said frequency converters for feeding said gathered tonesignals to the respective frequency converters.

6. A frequency conversion system for an electronic musical instrumentcomprising;

a signal source composed of at least one biased tone generatorgenerating a first tone signal having a first fundamental frequencycomponent which is biased from a normal musical scale by a predeterminedfrequency difference AfHz in a predetermined direction and furtherhaving exact harmonic components the frequencies of which are a positiveinteger multiple of the frequency of said first fundamental frequency;at least one frequency converter composed of a constant phase splitterconnected to said tone generator for splitting said first tone signalinto a pair of signals having a phase difference of 17/2 radians, anoscillator for generating a pair of frequency converting signals havingsaid predetermined frequency difference afHz and a phase difference of1r/2 radians, a first multiplier coupled to said phase splitter and saidoscillator for multiplying one output signal from said phase splitter byone of said frequency converting signals from said oscillator, a secondmultiplier coupled to said phase splitter and said oscillator formultiplying the other output signal from said phase splitter by theother of said frequency converting signals from said oscillator, andgathering means coupled to said multipliers for gathering'the outputsignals from said first and second multipliers, said frequency converterchanging all the frequency components of said first tone signal in adirection the opposite of said predetermined direction by saidpredetermined frequency difference Afl-lz in order to produce aconverted tone signal having frequency components consisting a secondfundamental frequency component and inexact harmonics thereof, saidsecond fundamental frequency component and said inexact harmonics havinga changed frequency which differs from said first fundamental frequencycomponent and said exact harmonics of said first tone signal by theconverting frequency of said frequency converter, respectively; and

an electro-acoustic transducer connected to said at least one frequencyconverter.

7. A frequency conversion system for an electronic musical instrumentcomprising;

a signal source composed of at least one biased tone generator which isdetuned from a normal musical scale by a predetermined frequencydifference Afl-lz in a predetermined direction for generating a firsttone signal having a first fundamental frequency component which isbiased from said normal musical scale by said frequency differenceAfl-lz in a predetermined direction and further having exact harmoniccomponents the frequencies of which are positive integer multiples ofthe frequency of said first fundamental frequency compo- 'nent;

at least one frequency converter composed of a modulator connected tosaid tone generator for modulating said first tone signal by amodulating signal having a frequency fcHz which is higher than twice thefrequencies of said exact harmonic components, a first filter coupled tosaid first modulator for passing only one side band of the modulatedsignal from said modulator, a further modulator coupled to said firstfilter for modulating the output signal from said filter by anothermodulating signal having frequency fcHz which differs by saidpredetermined frequency difference Afl-lz from said frequency fcHz, anda second filter coupled to said further modulator for passing only thelower side band of the modulated signal from said further modulator; and

i an electro-acoustic transducer connected to said second filter of saidfrequency converter.

1. A frequency conversion system for an electronic musical instrumentcomprising; a signAl source composed of at least one biased tonegenerator which is detuned from a normal musical scale by apredetermined frequency difference Delta fHz in a predetermineddirection for generating a first tone signal having a first fundamentalfrequency component which is biased from said normal musical scale bysaid frequency difference Delta fHz in said direction and which also hasexact harmonic components the frequencies of which are a positiveinteger multiple of the frequency of said first fundamental frequency;at least one frequency converter connected to said signal source forchanging all the frequency components of said first tone signal in adirection the opposite of said predetermined direction by a convertingfrequency equal to said predetermined frequency difference Delta fHz inorder to produce a converted tone signal having frequency componentsconsisting of a second fundamental frequency component and inexactharmonics thereof, each of said second fundamental frequency componentand said inexact harmonics having a changed frequency which differs fromsaid first fundamental frequency component and said exact harmonics ofsaid first tone signal by said converting frequency Delta fHz of saidfrequency converter; and an electro-acoustic transducer connected tosaid at least one frequency converter.
 2. A frequency conversion systemfor an electronic musical instrument as claimed in claim 1, wherein:said signal source consists essentially of twelve master oscillators andtwelve chains of frequency dividers corresponding to said twelve masteroscillators and being coupled thereto, said twelve master oscillatorsbeing biased to bias the frequency from the normal chromatic musicalscale in an upward direction; and said frequency converter connected tosaid signal source changes the frequency of an output signal from saidsignal source in a downward direction by said converting frequency forcompensating a pitch discrepancy from a natural pitch sensation of thehuman ear.
 3. A frequency conversion system for an electronic musicalinstrument as claimed in claim 1, wherein said tone generator has apitch selector which controls both the frequency of said tone generatorand said converting frequency.
 4. A frequency conversion system for anelectronic musical instrument as claimed in claim 1, wherein there is aplurality of tone generators corresponding in number to the respectivetones of the musical scale, and there is a plurality of frequencyconverters corresponding in number to the number of said tonegenerators, respectively.
 5. A frequency conversion system for anelectronic musical instrument as claimed in claim 1 further comprising akey switch system, and in which there is a plurality of tone generatorscorresponding in number to the respective tones of the musical scale,said key switch system being connected to said tone generator forselecting tone signals from among said plurality of tone generators andgathering said selected tone signals into a plurality of groups each ofwhich has successive tone signals of the musical scale, said frequencyconversion system having a plurality of frequency converterscorresponding in number to said plurality of groups of tone signals,said key switch system being connected to said frequency converters forfeeding said gathered tone signals to the respective frequencyconverters.
 6. A frequency conversion system for an electronic musicalinstrument comprising; a signal source composed of at least one biasedtone generator generating a first tone signal having a first fundamentalfrequency component which is biased from a normal musical scale by apredetermined frequency difference Delta fHz in a predetermineddirection and further having exact harmonic components the frequenciesof which are a positive integer multiple of the frequency of said firstfundamental frequency; at least one frequency converter composed of aconstant phase splitter connected to Said tone generator for splittingsaid first tone signal into a pair of signals having a phase differenceof pi /2 radians, an oscillator for generating a pair of frequencyconverting signals having said predetermined frequency difference AlphafHz and a phase difference of pi /2 radians, a first multiplier coupledto said phase splitter and said oscillator for multiplying one outputsignal from said phase splitter by one of said frequency convertingsignals from said oscillator, a second multiplier coupled to said phasesplitter and said oscillator for multiplying the other output signalfrom said phase splitter by the other of said frequency convertingsignals from said oscillator, and gathering means coupled to saidmultipliers for gathering the output signals from said first and secondmultipliers, said frequency converter changing all the frequencycomponents of said first tone signal in a direction the opposite of saidpredetermined direction by said predetermined frequency difference DeltafHz in order to produce a converted tone signal having frequencycomponents consisting a second fundamental frequency component andinexact harmonics thereof, said second fundamental frequency componentand said inexact harmonics having a changed frequency which differs fromsaid first fundamental frequency component and said exact harmonics ofsaid first tone signal by the converting frequency of said frequencyconverter, respectively; and an electro-acoustic transducer connected tosaid at least one frequency converter.
 7. A frequency conversion systemfor an electronic musical instrument comprising; a signal sourcecomposed of at least one biased tone generator which is detuned from anormal musical scale by a predetermined frequency difference Delta fHzin a predetermined direction for generating a first tone signal having afirst fundamental frequency component which is biased from said normalmusical scale by said frequency difference Delta fHz in a predetermineddirection and further having exact harmonic components the frequenciesof which are positive integer multiples of the frequency of said firstfundamental frequency component; at least one frequency convertercomposed of a modulator connected to said tone generator for modulatingsaid first tone signal by a modulating signal having a frequency fcHzwhich is higher than twice the frequencies of said exact harmoniccomponents, a first filter coupled to said first modulator for passingonly one side band of the modulated signal from said modulator, afurther modulator coupled to said first filter for modulating the outputsignal from said filter by another modulating signal having frequencyf''cHz which differs by said predetermined frequency difference DeltafHz from said frequency fcHz, and a second filter coupled to saidfurther modulator for passing only the lower side band of the modulatedsignal from said further modulator; and an electro-acoustic transducerconnected to said second filter of said frequency converter.